This invention relates generally to circuitry and methods for self-testing various elements in a graphic display device, more particularly to self-test by the system of system elements between the frame buffer memory DRAMs and the inputs to a graphic display monitor.
It is becoming common for graphics systems to test themselves without the aid of external fixtures or human observation. This includes both power-on self-test and extended diagnostics of digital circuitry in the graphics system. Most of the system up to and including the frame buffer can be tested without the use of external test instrumentation or feedback from an observer.
One exception has been the display of video information on the monitor. Hardware from the video shift registers to the CRT electron gun has only been testable by viewing the resultant displays on the CRT screen.
The frame buffer itself can be tested by writing test data into the frame buffer and then reading back the test data and comparing it with the input test data. Although the color map itself is testable, it is hard, however, to test the video shift registers feeding the color map and the data path between the color map and the DACs. In normal system operation, there is only a one directional data path out of the frame buffer through the color map and DACs and so its signals cannot be read from the system's processor. It is also hard to test the signals at the outputs of the DACs by use of external test instrumentation. In prior systems, everything from the frame buffer outputs downstream to the CRT display, except the color map RAM, is not automatically testable. Its operation can only be observed by looking at the CRT display and working back manually.
Accordingly, a need remains for a better way to test elements of a graphics system between the frame buffer and the CRT display monitor.